Conventional shaft couplings used for the purpose as described above include a flexible coupling utilizing resiliency of a rubber material or coil springs. A slide coupling such as an Oldham's coupling has also been used heretofore.
However, the coupling using the resilient members such as rubber members are liable to be distorted in the rotational direction due to a load torque and they are not suitable for uses which require highly accurate transmission of a rotational angle. The coupling using coil springs has such a disadvantage that the allowable range of an axial offset is narrow and the fluctuation of the rotational force due to an increase of a bearing load is large. On the other hand, the slider coupling is disadvantageous especially in intermittent rotational driving since an impact load is directly transmitted therethrough, causing or promoting undesirable backlash.